The Open System Interconnection (OSI) reference model describes how information from a software application in one device moves through a network medium to a software application in another device. The OSI reference model is a conceptual model composed of seven layers, each specifying particular network functions. The OSI model divides the tasks involved with moving information between networked devices into seven smaller, more manageable task groups. A task or group of tasks is then assigned to each layer of the OSI model. The uppermost layer is the application layer followed by the presentation layer, session layer, transport layer, network layer, data link layer and the physical layer. Each layer is reasonably self-contained so that the tasks assigned to each layer can be implemented independently. This enables the solutions offered by one layer to be updated without adversely affecting the other layers. Standard communication models conceptually based on the OSI model include TCP/IP, SS7 (Signaling System #7), AppleTalk, SNA (Systems Network Architecture), DSL (Digital Subscriber Line), UMTS (Universal Mobile Telecommunications System), etc.
Each layer of a communication model is defined by a specific structure and corresponding protocol. The structure determines how information is arranged or organized at a particular layer, often referred to as a data unit. For example, information is organized as frames at the data link layer, packets at the network layer, segments or datagrams at the transport layer and as data at the session, presentation and application layers in the OSI model. Actual communication is made possible by using communication protocols. In the context of data communication, a protocol is a formal set of rules and conventions that governs how devices exchange information over a communication medium. A protocol implements the functions of one or more of the OSI layers.